The invention relates to a method for the removal of water from a paper or board web, by hot-pressing, the hot-pressing taking place when the web is in direct contact with a heated roll or cylinder face. According to the method, the web to be pressed is passed, when supported by a pressed fabric, through at least two separate press stages.
The most common prior art method of dewatering of fibrous webs, in particular, paper and board webs, is to pass the web through a press nip formed by two rolls placed one opposite the other. As is well known, one or two press fabrics are used in the dewatering nips, and these carry the water removed from the web further and act as a fabric carrying the web forward.
With increasing production rates of paper machines, the dewatering performed as nip pressing has become a bottleneck that limits the increase in the running speeds, because the press nips formed by a pair of rolls have a short area, so that with high speeds, the time or dwell of the web in these press nips remains short.
Attempts can be made to extend the compression area in roll-nips by using rolls of larger diameter and soft press fabrics. However, even with these means, the limit of an economically satisfactory embodiment is soon reached. As a result of these problems, and for other reasons, so-called extended-nip presses have been invented in recent years. Reference may be made in this respect by way of example to U.S. Pat. Nos. 3,783,097; 3,808,092; 3,840,429; 3,970,515; 4,201,624; and 4,229,253, as well as Finnish Patents to Nos. 65,104; 70,952; and 71,369.
Extended-nip presses are known in the prior art wherein the press area is formed between a revolving hose mantle, provided with a stationary core and a counter-roll. In the stationary roll core, hydraulically loaded pressure shoes are utilized by means of which, with the intermediate of the revolving mantle, a compression pressure is applied to the web towards the counter-roll. The counter-roll may be either an ordinary press roll or a roll provided with a hydraulic glide shoe. As examples of the prior art extended-nip press of the type described above, reference is made to British Patent Application No., 2,057,027, to be published as Patent Application WO 82/02567, and to Finnish Patent No. 66,932. So called hot-pressing methods are also known in the prior art, and reference is made by way of example to U.S. Pat. No. 4,324,613, and to U.S. patent application Ser. No. 07/187,259 filed on Apr. 22, 1988, now U.S. Pat. No. 4,976,820.